This invention relates generally to chemical mechanical polishing of substrates, and more particularly to a method and apparatus for detecting a polishing endpoint in chemical mechanical polishing.
An integrated circuit is typically formed on a substrate by the sequential deposition of conductive, semiconductive or insulative layers on a silicon wafer. After each layer is deposited, the layer is etched to create circuitry features. As a series of layers are sequentially deposited and etched, the outer or uppermost surface of the substrate, i.e., the exposed surface of the substrate, becomes increasingly non-planar. This non-planar surface presents problems in the photolithographic steps of the integrated circuit fabrication process. Therefore, there is a need to periodically planarize the substrate surface.
Chemical mechanical polishing (CMP) is one accepted method of planarization. This planarization method typically requires that the substrate be mounted on a carrier or polishing head. The exposed surface of the substrate is placed against a rotating polishing pad. The polishing pad may be either a xe2x80x9cstandardxe2x80x9d pad or a fixed-abrasive pad. A standard pad has a durable roughened surface, whereas a fixed-abrasive pad has abrasive particles held in a containment media. The carrier head provides a controllable load, i.e., pressure, on the substrate to push it against the polishing pad. A polishing slurry, including at least one chemically-reactive agent, and abrasive particles if a standard pad is used, is supplied to the surface of the polishing pad.
The effectiveness of a CMP process may be measured by its polishing rate, and by the resulting finish (absence of small-scale roughness) and flatness (absence of large-scale topography) of the substrate surface. The polishing rate, finish and flatness are determined by the pad and slurry combination, the carrier head configuration, the relative speed between the substrate and pad, and the force pressing the substrate against the pad.
In order to determine the effectiveness of different polishing tools and processes, a so-called xe2x80x9cblankxe2x80x9d wafer, i.e., a wafer with one or more layers but no pattern, is polished in a tool/process qualification step. After polishing, the remaining layer thickness is measured at several points on the substrate surface. The variations in layer thickness provide a measure of the wafer surface uniformity, and a measure of the relative polishing rates in different regions of the substrate. One approach to determining the substrate layer thickness and polishing uniformity is to remove the substrate from the polishing apparatus and examine it. For example, the substrate may be transferred to a metrology station where the thickness of the substrate layer is measured, e.g., with an ellipsometer. Unfortunately, this process can be time-consuming and thus costly, and the metrology equipment is costly.
One problem in CMP is determining whether the polishing process is complete, i.e., whether a substrate layer has been planarized to a desired flatness or thickness. Variations in the initial thickness of the substrate layer, the slurry composition, the polishing pad material and condition, the relative speed between the polishing pad and the substrate, and the load of the substrate on the polishing pad can cause variations in the material removal rate. These variations cause variations in the time needed to reach the polishing endpoint. Therefore, the polishing endpoint cannot be determined merely as a function of polishing time.
One approach to determining the polishing endpoint is to remove the substrate from the polishing surface and examine it. If the substrate does not meet the desired specifications, it is reloaded into the CMP apparatus for further processing. Alternatively, the examination might reveal that an excess amount of material has been removed, rendering the substrate unusable. There is, therefore, a need for a method of detecting, in-situ, when the desired flatness or thickness had been achieved.
Several methods have been developed for in-situ polishing endpoint detection. Most of these methods involve monitoring a parameter associated with the substrate surface, and indicating an endpoint when the parameter abruptly changes. For example, where an insulative or dielectric layer is being polished to expose an underlying metal layer, the coefficient of friction and the reflectivity of the substrate will change abruptly when the metal layer is exposed.
In an ideal system where the monitored parameter changes abruptly at the polishing endpoint, such endpoint detection methods are acceptable. However, as the substrate is being polished, the polishing pad condition and the slurry composition at the pad-substrate interface may change. Such changes may mask the exposure of an underlying layer, or they may imitate an endpoint condition. Additionally, such endpoint detection methods will not work if only planarization is being performed, if the underlying layer is to be over-polished, or if the underlying layer and the overlying layer have similar physical properties.
In view of the foregoing, there is a need for a polishing endpoint detector which more accurately and reliably determines when to stop the polishing process. There is also a need for an means for in-situ determination of the thickness of a layer on a substrate during a CMP process.
In one aspect, the invention is directed to a chemical mechanical polishing apparatus to polish a substrate having a first surface and a second surface underlying the first surface. The apparatus has a first polishing station with a first optical system, a second polishing station with a second optical system, at least one processor. The first optical system including a first light source to generate a first light beam to impinge the substrate as it is polished at the first polishing station, and a first sensor to measure light from the first light beam that is reflected from the first and second surfaces to generate a first interference signal. The second optical system includes a second light source to generate a second light beam to impinge on the substrate as it is polished at the second polishing station, and a second sensor to measure light from the second light beam that is reflected from the first and second surfaces to generate a second interference signal. The first light beam has a first effective wavelength, and the second light beam has a second effective wavelength that differs from the first effective wavelength. The processor determines a polishing endpoint at the first and second polishing stations from the first and second interference signals, respectively.
Implementations of the invention may include the following features. The first effective wavelength may be greater than the second effective wavelength. The second light beam may have a second wavelength, e.g., between about 400 and 700 nanometers, that is shorter than a first wavelength, e.g., between about 800 and 1400 nanometers, of the first light beam. A third polishing station may have a third optical system which includes a third light source to generate a third light beam to impinge on the substrate as it is polished at the third polishing station, and a third sensor to measure light from the third light beam that is reflected from the first and second surfaces to generate a third interference signal. The third light beam may have a third effective wavelength that is equal to or smaller than the second effective wavelength. A carrier head may move the substrate between the first and second polishing stations. Each polishing station may include a rotatable platen with an aperture through which one of the first and second light beams can pass to impinge the substrate. Each polishing station may also include a polishing pad supported on a corresponding platen, each polishing pad having a window through which one of the first and second light beams can pass to impinge the substrate.
In another embodiment, the invention is directed to a method of chemical mechanical polishing. In the method, a substrate is polished at a first polishing station, a first interference signal is generated by directing a first light beam having a first effective wavelength onto the substrate and measuring light from the first light beam reflected from the substrate, and a first endpoint is detected from the first interference signal. After detection of the first endpoint, a second interference signal is generated by directing a second light beam having a second effective wavelength onto the substrate and measuring light from the second light beam reflected from the substrate, and a second endpoint is detected from the second interference signal. The second effective wavelength differs from the first effective wavelength.
Advantages of the invention include the following. With two optical systems, an estimate of the initial and remaining thickness of the layer on the substrate can be generated. Employing two optical systems operating at different effective wavelengths also allows more accurate determination of parameters that were previously obtained with a single optical system.
Other features and advantages of the invention will become apparent from the following description, including the drawings and claims.